According to Homeland Security spokesmen, the border wall is to be as tall as 30 feet high, but no shorter than 18 feet, in order to deter illegal immigrants looking to cross the border. The wall is to also be at least six feet beneath the ground, allegedly to prevent tunneling underneath it.

Additionally, the structure must take at least an hour, preferably four hours, to punch a hole through the wall. In theory, this is to provide enough time for border patrol agents to respond and prevent them from breaking through.

The wall must also be able to withstand a “sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools.”

The contracting documents don’t give a hint as to the eventual scope of the wall, but Mr. Trump’s budget suggested it would in fact run the nearly 2,000 miles of the border. And the documents did call for designs that would work on a 45-degree slope, indicating a desire to build fencing even along mountainous areas where the border patrol had previously thought barriers unnecessary.

Currently about 654 miles of the border have a barrier — 354 miles of that is a type of pedestrian fence, while the other 300 miles are vehicle barriers designed to stop cars and trucks, but which easily let people and animals through.

The existing pedestrian fencing varies in design, but types of fence that have alternative slats allowing Border Patrol agents to see through to the Mexican side are preferred because they help cut down on ambushes by rock-throwers, according to a recent government audit.

Nearly 700 companies have registered as potential vendors for the wall project. Contractors have been given until March 29 to submit proposals for the first phase of the build.